1. Field of the Invention
The present invention relates to a manufacturing method of thin film semiconductor substrates. The present invention specifically relates to a manufacturing method of a thin film semiconductor substrate including an ion implantation and cleaving process.
2. Description of the Related Art
A thin film semiconductor substrate typically represented by an SOI (silicon on insulator) substrate has a single crystal semiconductor thin film, for example, of a single crystal silicon, on an insulating substrate. A semiconductor device manufactured using such a thin film semiconductor substrate has sufficient insulation for semiconductor elements along its depth direction. Accordingly, such substrates are appreciated as being useful in manufacturing semiconductor devices requiring high integration and high functionality.
The ion implantation and cleaving process (so-called “smart cut method”, registered trademark) is known as a method for manufacturing thin film semiconductor substrates. In this method, first, ions of a light mass element, such as hydrogen, are implanted at a specified depth into a wafer made of a single crystal silicon for an activation layer. Next, an insulating substrate is laminated on an ion implantation side of the wafer for the activation layer. Then, bubbles are generated in the ions introduced into the wafer for the activation layer by causing a change of volume in minute cavities by a heat treatment at a temperature in a range of 400° C. to 700° C. By this process, a bubble layer is formed at the specified depth in the wafer for the activation layer. After formation of the bubble layer, the wafer for the activation layer is cleaved along the bubble layer, which functions as a cleaving surface, thereby obtaining a thin film semiconductor substrate that includes a semiconductor thin film (single crystal silicon thin film) made of the thin wafer for the activation layer on the side of the insulating substrate (refer to Japanese Patent Application Laid-Open No. 2009-158943, specifically the paragraphs 0039 to 0041).
To achieve a high integration density, which includes forming semiconductor elements in a high density on the thin film semiconductor substrate, it is desirable that the thin film has a uniform thickness and the cleaving surface has high flatness. Therefore, the semiconductor elements are formed on the thin film substrate after subjecting the surface (the cleaving surface) of the semiconductor thin film to flattening by polishing.
Meanwhile, in the ion implantation and cleaving process, merging of bubbles takes place during the heat treatment that is performed at the temperature in the range of 400° C. to 700° C. Diameters of bubbles in the bubble layer increase to around 10 nanometers (nm) due to merging of the bubbles. The cleaving surface whose form depends on the shapes of the inner walls of the bubbles becomes uneven when the bubbles have larger diameters. Especially, when manufacturing a semiconductor thin film having a certain high thickness, sufficiently large implantation energy is required in the ion implantation process to introduce ions into a specified depth of the wafer for the activation layer. Large implantation energy leads to a widening of the implantation distribution range of the light mass element ions and an increase in the thickness of the bubble layer, and this makes the cleaving surface still more uneven.
An increase in the unevenness of the cleaving surface increases the polishing work for flattening the cleaving surface and causes a longer TAT (turn around time) and decreases the yield.
The present invention aims to provide a thin film semiconductor substrate manufacturing method that enables forming of the bubble layer with minute bubbles, and as the effect, improves the TAT and the yield.